Pyrithione sources, such as sodium pyrithione or zinc pyrithione (hereinafter, ZPT) have been known widely used to provide an antimicrobial efficacy for a broad list of industrial applications. For example, sodium pyrithione and zinc pyrithione have been employed as preservatives against the growth of micro-organism in personal care compositions, paints, adhesives and so on. Another application of pyrithione source, for example, is the wide use of ZPT as an antidandruff agent in hair care products.
Bar soap is a popular product form for cleansing. A bar soap comprising pyrithione source for antimicrobial efficacy can be desired. However, during manufacturing, handling or storage of a bar soap, various metallic parts of the manufacturing equipment, for example pipes, nozzles may be contacted with the bar soap. In some situation, such contact can maintain a long time (e.g. overnight to 24 hours), and at a relatively elevated temperature.
Such contact has the potential of causing a color change of the bar soap, so called “discoloration”, which is from a colored precipitate. The precipitate is a reaction resultant of pyrithione source with dissolved metal ions, such as ferric ions and/or cupric ions from the metal parts of the equipment. The discoloration can also be brought about by dissolved metal ions as metal contaminants in raw materials used for making bar soap, such as fatty esters.
In one aspect, this discoloration of a bar soap is unwanted for aesthetic reasons, where certain desirable colors are important to meet the consumer's needs. Consumer dissatisfaction and economic loss can result from such discoloration. In another aspect, the formation of a colored pyrithione precipitate is unwanted for a safety and regulatory consideration. For example, a green discoloration from copper pyrithione may stain stratum corneum, resulting a bar soap that is not suitable for body cleaning or facial cleaning.
In the past, a number of solutions toward this pyrithione discoloration problem have been described. For example, in U.S. Pat. No. 4,161,526, JP Patent Publication 2001-278863A, U.S. Pat. Nos. 4,482,715, 4,957,658 and 4,818,436, a number of materials including zinc-containing materials, borates, reducing agents selected from alkali metal sulfite, alkali metal bisulfites, hydrazine and mixtures thereof and HEDP have been used to address the pyrithione discoloration problem.
However, those solutions may be less desired for safety reason, for regional regulatory reasons, and/or for cost reasons and the like.
Specifically, the use of zinc-containing compounds may be less desired in solving this discoloration problem because: 1) soluble zinc level limitations from regional regulatory authorities, as there are countries like China that limit the maximum soluble zinc salt level excluding those zinc contributed by ZPT or zinc phenolsulfonate to be 1% in a cosmetic composition and 2) adoption of such zinc materials may not be very cost effective.
As to the use of reducing agents, they tend to interact with other ingredients in bar soap, thus posing challenges in formulation and process.
As to the use of HEDP, the amount of HEDP chelant incorporated in a bar soap needs to be very carefully controlled in order not to chelating some metal ions which is useful in bar soap formulation.
Hence, there still exists a need for a bar soap comprising a pyrithione source, which is free of discoloration problem, is cost effective and is free of the shortcomings of discoloration prevention approaches in the art mentioned above. The present invention provides such solutions.